1. Field of the Invention
The present invention relates generally to apparatus and methods used to remove contaminants from the surface of a chosen substrate by application of a stream or jet spray of expanded fluid to the substrate surface. Such apparatus and methods are typically used to remove trace contaminants from delicate surfaces or precision hardware found in high sensitivity optical, electronic and precision mechanical equipment. More particularly, the present invention relates to an improved jet spray system which controls electrostatic charges that may be generated on or near the substrate surface.
2. Description of Related Art
It is essential that the delicate and precision surfaces of optical-mechanical-electronic equipment be contaminant-free. For example, infrared optical sensor systems use thin film optical coatings to enhance in-band transmittance and reject out-of-band radiation. Even trace amounts of contamination degrade spectral performance by scattering, absorbing, or reflecting incident radiation.
The particulate and molecular contaminants present on optical surfaces can cause problems by changing emittance characteristics thereby increasing thermal noise in optical devices. Dust and debris contamination present on a detector surface may also generate clutter and the appearance of false targets. Further, the presence of molecular contaminants on precision optical equipment surfaces, such as lenses, windows and optical filters, results in the absorption and scatter of incident energy with the resultant decrease in system quality.
In precision computer and gyroscope equipment, particulate contamination in a moving disk drive or in a spinning gyroscope stator may damage or disable such a system beyond repair. Technologies used in manufacturing integrated circuits similarly require substrates be free of particulate and molecular contaminants.
A wide variety of cleaning solvents and solutions, such as chlorofluorocarbons and ketones, have been utilized to clean delicate surfaces. However, the majority of these cleaning solutions and solvents are environmentally hazardous. In response to these environmental concerns, a number of environmentally safe cleaning mediums have been proposed. One particular cleaning medium which has been investigated is carbon dioxide. A variety of investigators have reported cleaning various parts utilizing a stream or jet spray of carbon dioxide. The jet stream of carbon dioxide, also known as "dry ice snow", has been used to remove light oils and particulate contaminants from surfaces. Such carbon dioxide jet sprays have been used to clean contaminants from a variety of surfaces including silicon wafers, telescope mirrors and thin film optical coatings.
Since the carbon dioxide snow is very cold, around minus 60 degrees centigrade at ambient pressure, prolonged spray will cool the substrate surface causing a build up of condensation on the substrate after application of the stream or spray of carbon dioxide snow to the substrate is discontinued. A variety of investigators have used a dry nitrogen gas purge to eliminate condensation of moisture on the substrate surface. The dry nitrogen gas can be applied to the substrate surface before, during and after completion of the cleaning process to prevent the build up of condensation.
The formation and use of carbon dioxide snow streams is disclosed in the following articles, the contents of which are hereby incorporated by reference: S. A. Hoening, "Cleaning Surfaces With Dry Ice", Compressed Air Magazine, August 1986; R. V. Peterson, C. W. Bowers, "Contamination Removal By CO.sub.2 Jet Spray," SPIE, Volume 1329, Optical System Contamination, Effects, Measurements, Control II, 1990; L. Layden, D. Wadlow, "High Velocity Carbon Dioxide Snow For Cleaning Vacuum System Surfaces," Journal of Vacuum Science Technology, A8 (5), September/October 1990; R. Sherman, W. H. Whitlock, "The Removal of Hydrocarbons and Silicon Grease Stains From Silicon Wafers," Journal of Vacuum Science Technology, B8 (3) May/June 1990; R. Zito, "Cleaning Large Optics With CO.sub.2 Snow," SPIE, Volume 1236, Advanced Technology Optical Telescopes IV, 1990.
The use of carbon dioxide snow to clean delicate surfaces has proven to be very useful. However, as with any new cleaning system, there is a continual need to further develop and improve upon existing systems and procedures.